1. Field of the Disclosure
The present disclosure relates generally to a method and apparatus for measuring a zeta potential of suspended particles. In particular, the apparatus is a zeta probe coated with a conducting and transparent thin film, wherein the film provides a path for ionic current to flow allowing an electric field to be applied between the probe and a counter electrode without surface charging. This probe configuration allows for the use of a high frequency AC electric field thereto which allows for the measuring of the magnitude of the interaction between the particle Brownian motion and the particle AC field motion and allows for the use of a DC field thereto which allows for the measuring of the sign of the zeta potential.
2. Discussion of the Background Art
Zeta potential is a measure of the charge on suspended particles and is a critical parameter in determining the stability of particle suspensions. Suspended particles with the same sign charge repel each other preventing particles from joining in agglomeration. The magnitude of the charge determines the degree of stability of the suspension. A means of determining the particle charge utilizes the response of particles to an applied electric field. Under the influence of an electric field charged particles move in response to the electric field force on the charged particle. In equilibrium with the viscous force of the suspending fluid the charged particles drift in the direction of the force at a drift velocity which is dependent on the charge, electric field and the viscosity of the suspending medium. A measurement of the drift velocity provides a means of determining the particle charge and the Zeta Potential.
U.S. Pat. Nos. 5,094,526 and 6,396,979 (which are incorporated by reference herein in their entirety) disclose an optical means of determining particle velocity. An optical probe inserted into a particle suspension delivers an optical beam into the suspension. The optical beam is backscattered by the suspended particles and the backscattered light is collected by the probe and transmitted by fiber optics to a photo detector. Also transmitted to the detector is a component of the optical beam which is Fresnel reflected from the probe suspending medium interface. The light scattered from the particles is Doppler frequency shifted due to the fact that the particles are in Brownian motion. The mixing of the frequency shifted scattered light with the reflected unshifted light at the photo detector results in a component of the photo detector output which is the difference frequency between the scattered light and the reflected light. The distribution of frequency shifts are measured as a frequency power spectrum and represent the distribution of velocities of the particles in Brownian motion. It is the purpose of the aforementioned patents to collect the distribution of the frequency shifts representing the distribution of velocities of the particles. From the distribution of velocities the size distribution of the suspended particles is calculated using the theories of dynamic light scattering. Disclosed in the aforementioned patents are the means of fabricating the optical probe which combines the scattered and reflected light, the use of a miniature optical element to transmit into the suspension and focus that light at the outer surface of a probe interface window, and the use of a window material to enhance the reflection at the interface.
Prior art shows how to optically measure particle motion and to measure that motion induced by the application of an electric field to suspended particles. Generally the particles are observed through a transparent wall in a sample containing cell. The motion is determined by measuring the Doppler Displacement of the optical beams light frequency. From the particles velocity, electric field and the fluid viscosity the particle electrophoretic mobility and charge can be calculated. Such a measurement is limited to optical paths within the measuring volume of the suspension which are not going to excessively attenuate the optical beam or multiscatter the scattered light. The prior art restricts the suspension to be very dilute or provides a complex external beam focusing to position the viewing volume close to a surface of the sample containing cell wall. Further the viewing volume must be positioned within the sample containing cell at a point which has negligible or non-interfering electroosmotic fluid flow.
An example of a prior art electrophoretic mobility measuring apparatus is disclosed in U.S. Pat. No. 7,449,097 (Sekiwa et al.) which is capable of conducting measurement with high sensitivity with optical attenuation reduced by incidence of light through the electrode face. This apparatus comprises a transparent electrode forming a part of a cell wall of a cell capable of confining a sample, and the other electrode opposite to the transparent electrode. A voltage is applied across these electrodes, and light is incident upon the inside of the cell through the transparent electrode. The scattering light which scatters from a sample S at a predetermined angle with respect to the incident angle, is received through the transparent electrode. The Doppler displacement is then measured based on the difference in frequency between the incident light and the outgoing light.
The problem associated with these prior art methods is that they measure particle velocity directly. In addition, direct current (DC) systems need to go into the solution to obtain an accurate measurement, need high electric fields to obtain an accurate reading, and ions in the solution tend to interfere with the DC reading. Alternating circuits (AC) operate at low frequency, but still exhibits some electro-chemical problems.
The present disclosure overcomes all of the electro-chemical interaction and problems exhibited by conventional electrophoretic mobility measuring apparatuses by the utilization of high frequency alternating current and by avoiding measurement of suspending fluid flow. In addition, the present disclosure provides a more accurate analysis than conventional zeta potential meters, requires no correction for electo-osmonic flow, and can measure zeta potential at higher concentrations, whereas conventional cells must first dilute before measuring.